Science - USA (2021-07-16)

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SCIENCE 16 JULY 2021 • VOL 373 ISSUE 6552 259



ast month, at the G7 Leaders’ Summit in Corn-
wall, United Kingdom, the leading industrial na-
tions addressed the sustainable and safe use of
space, making space debris a priority and calling
on other nations to follow suit. This is good news
because space is becoming increasingly congest-
ed, and strong political will is needed for the in-
ternational space community to start using space sus-
tainably and preserve the orbital environment for the
space activities of future generations.
There are more than 28,000 routinely tracked objects
orbiting Earth. The vast majority (85%) are space debris
that no longer serve a purpose. These debris objects are
dominated by fragments from the approximately 560
known breakups, explosions, and collisions of satellites
or rocket bodies. These have left behind an estimated
900,000 objects larger than 1 cm and a staggering 130
million objects larger than 1 mm in
commercially and scientifically valu-
able Earth orbits.
Today’s already active satellite infra-
structure provides a multitude of criti-
cal services to modern society, including
communication, weather, navigation,
and Earth-monitoring missions. Its loss
would severely damage modern society.
Furthermore, a new era in space has
just started, driven by commercial, low-
latency broadband services that rely on
large constellations of satellites in low
Earth orbit. These will revolutionize connectivity on the
ground and in the air. However, they will also increase
space traffic. The satellites to be launched over the next 5
years will surpass the number launched globally over the
entire history of spaceflight. Congestion in space is only
going to get worse.
It is apparent that debris mitigation strategies—de-
fined two decades ago by experts in the world’s leading
space agencies—are ever more important. They aim to
prevent explosive breakups by venting residual energy
from space systems at the end of their missions, and to
“dispose” of a space object through a final maneuver
that causes it to reenter Earth’s atmosphere. Although
these strategies are widely recognized, dozens of large
space objects are still stranded every year in criti-
cal orbital regions where they will remain for several
hundred years. And an average of eight fragmentation
events in orbit occur annually, adding more pollution
and increasing the likelihood of more collisions. Opera-
tions in space are themselves facing the burden of in-
creasing evasive maneuvers to prevent losing a mission.

In the most densely populated orbital altitudes, space
objects are receiving dozens of collision warnings per
day, of which only the most critical can be avoided. The
number of such alerts will grow as large constellations
of satellites come online.
Another important facet of the debris problem is
the risk on Earth from reentering objects. Between 100
and 200 metric tons of human-made hardware reen-
ters Earth’s atmosphere every year in an uncontrolled
fashion. Heat-resistant material, like titanium or stain-
less steel, can survive the harsh reentry conditions.
Progress can be made by advancing technology to
ensure spaceflight safety. For example, the European
Space Agency’s Space Safety Programme is developing
solutions that make disposal and energy passivation
actions more fail-safe. “Deorbiting kits” will provide
redundant propulsion and communication to ensure
disposal of a spacecraft even after
it ceases to function. A new field of
“design-to-demise” will aim to replace
critical components with less heat-re-
sistant material to limit their chance
of reaching ground upon reentry. In
addition, a more systematic deploy-
ment of ground-based laser tracking
could increase the accuracy of space
surveillance data and consequently
limit the number of collision avoid-
ance alerts. Laser power could even
transfer a small amount of momen-
tum to objects to prevent their collisions. On top of
that, missions, such as Clearspace-1, will aim to re-
move targeted debris through robotic capture.
An internationally binding regime for the man-
agement of debris and space traffic is pending. Thus
far, space missions have been supervised on the na-
tional level only, and states have been encouraged to
translate the nonbinding space debris guidelines into
national regulations. Space, however, is a commonly
used resource with a limited capacity. International
harmonization of space traffic would be required for
an efficient and interference-free use of space. The co-
ordinated use of the available radio frequencies could
serve as a template. Furthermore, the implementation
of space debris mitigation requirements should be
tracked, following internationally binding principles.
New and affordable technical solutions might stimu-
late more ambitious steps in international regulation
to preserve space for the spacefarers of tomorrow.

–Holger Krag

A sustainable use of space

Holger Krag
is head of the
Space Safety
Programme at the
European Space
Agency, Darmstadt,
Germany. holger.



in space is

only going

to get worse.”

0716Editorial.indd 259 7/13/21 5:30 PM

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